This invention relates to a method of purifying hexafluoroacetone hydrate containing hydrates of chlorofluoroacetones as impurities.
Hexafluoroacetone (abbreviated to HFA) is known as an important intermediate for the preparation of some fluorine-containing polymers and a variety of useful organic compounds including medicines and agricultural chemicals. HFA is a toxic gaseous compound of which the boiling point is -28.degree. C. and, therefore, must be handled very cautiously when storing and transporting.
HFA readily reacts with water to form a monohydrate, which is a solid having a melting point of 46.degree. C., as represented by Equation (1). ##STR1## The monohydrate easily dissolves in excess water to undergo further hydration. When the mole ratio of water to HFA is about 3, the hydrate (hereinafter referred to as "HFA.3H.sub.2 O") is a constant boiling mixture of which the boiling point is 106.degree. C. HFA.3H.sub.2 O is a stable liquid easy to store and transport, and anhydrous HFA can easily be recovered from HFA.3H.sub.2 O by a simple dehydration reaction using a suitable dehydrating agent such as concentrated sulfuric acid or phosphorus pentoxide. Besides, HFA.3H.sub.2 O itself has unique properties as a solvent. Therefore, HFA hydrate is attracting increasing attention.
Usually HFA is prepared by reaction of hexachloroacetone with hydrogen fluoride gas in the presence of catalyst such as chromium trifluoride or dichromium trioxide. The gaseous product of the reaction contains unreacted hydrogen fluoride and by-produced hydrogen chloride, and these hydrogen halides tend to react with HFA to form complexes. According to U.S. Pat. No. 4,386,223, the hydrogen halides can be removed by first hydrating the crude HFA gas and then neutralizing the hydrogen halides in the hydrated material by using a calcium compound such as CaCO.sub.3, Ca(OH).sub.2 or CaO as a neutralizing agent.
In the preparation of HFA from hexachloroacetone it is inevitable that the crude HFA gas contains some amounts of organic by-products, which are principally chlorofluoroacetones (hereinafter referred to as "FK's") such as chloropentafluoroacetone (hereinafter referred to as "5FK"), dichlorotetrafluoroacetone (referred to as "4FK") and trichlorotrifluoroacetone (referred to as "3FK"). Since these FK's are high in toxicity and, like HFA, also in reactivites it is desirable to completely separate the FK's from HFA. Like HFA, most of these FK's tend to form complexes with hydrogen halides and, moreover, readily react with water to form their respective hydrates. It is difficult to separate such FK's hydrates from HFA hydrate because the FK's hydrates have boiling points ranging from about 105.degree. C. to about 106.degree. C., which are very close to the boiling point of HFA.3H.sub.2 O. The aforementioned U.S. Pat. No. 4,386,223 proposes to decompose the FK's hydrates coexisting with HFA hydrate by using an alkali metal carbonate, alkaline earth metal hydroxide or alkaline earth metal oxide as a decomposing agent.
In theory it is possible to separate FK's from HFA, without hydrating them, by distillation utilizing differences in boiling points of the respective fluoroacetones, but in practice this method is disadvantageous in some respects such as the need of pressure distillation and the intricateness of the process.
A major cause of the difficulty in purifying HFA is that HFA itself exhibits high reactivities. Besides the reactions with hydrogen halides to form complexes, HFA readily undergoes a decomposition reaction with an alkali compound such as KOH or NaOH. As homologous compounds FK's also exhibit nearly similarly high reactivities, so that it has been accepted as to be very difficult to separate FK's from HFA by utilizing a certain difference in chemical reactivities. For example, separation or removal of FK's from crude HFA gas by decomposition with an alkali hydroxide such as NaOH is impractical because a decomposition reaction of HFA takes place simultaneously and proceeds competitively so that a large amount of HFA is lost during complete decomposition of the FK's.